A multistage vacuum pump in which a plurality of pump chambers are connected in series is widely used conventionally.
As an example, FIG. 10 shows an internal structure of a multistage vacuum pump. In this vacuum pump, a shaft 71 and a shaft 72 are disposed in parallel inside a housing, not shown in the drawing. Rotors 51 to 54 attached to the shaft 71 and rotors 61 to 64 attached to the shaft 72 form respective pairs which rotate without contacting each other in pump chambers partitioned by partition plates, not shown in the drawing.
In the drawing, second stage rotors 52, 62, third stage rotors 53, 63, and fourth stage rotors 54, 64 from an upstream side of a gas flow direction are claw type rotors. With claw type rotors, a male rotor and a female rotor rotate in opposite directions to each other such that gas suctioned through a suction port is compressed by variation in a volume of an enclosed space between the two rotors and the housing. Thus, a vacuum subject space connected to the suction port is set in a vacuum state. A vacuum pump using claw type rotors is disclosed in Patent Document 1 (Japanese Patent Application Publication No. 2008-88879), for example.
FIG. 11 is a view illustrating intake, compression, and discharge processes of a conventional multistage vacuum pump. As shown in FIG. 11A, at intake and discharge end points, an intake port 73 and a discharge port 74 are closed by the male rotor 52 and the female rotor 62, but when the two rotors 52, 62 are rotated further, as shown in FIG. 11B, the intake port 73 is opened such that gas is taken in and a compression pocket 75 is compressed. When the discharge port 74 is opened, the compressed gas is discharged and transferred to a subsequent stage pump chamber. When the two rotors 52, 62 rotate further following the completion of discharge, as shown in FIG. 11C, intake starts again, followed by the compression process.
In this type of multistage vacuum pump, the gas compressed in a previous stage compression pocket is transferred to the subsequent stage through a gas passage that opens as the rotors rotate. Timings of the compression process and this opening operation are set mainly on the basis of a desired compression volume ratio in the previous stage.    Patent Document 1: Japanese Patent Application Publication No. 2008-88879
In a multistage vacuum pump, the rotors are typically fixed by key grooves or the like formed in the shafts. In this case, to prioritize ease of manufacture, the male rotors and the female rotors of all stages are disposed alternately either coaxially and at identical angles or simply at identical angles so that no phase differences occur between the respective stages.
However, gas is pumped between the respective stages at an identical timing in all of the stages, leading to increases in pulsation and power variation. Accordingly, noise and vibration increase, and it becomes necessary to provide a large power supply capable of absorbing a power variation peak. As a result, an increase in cost occurs.